The electrical furnaces used in the preparation of silicon are arc furnaces provided with graphite electrodes. Because of the very high temperatures and the flows of hot air inside such furnaces, a certain proportion of silicon escapes with the flue gases and oxidizes into silica fumes.
These fumes of silica, i.e. of SiO.sub.2, have very small grain size (in the order of a few .mu.m) and they are always black in color. This color is due essentially to the silica particles capturing carbon that comes from wear of the graphite electrodes.
The black color of these silica fumes makes them unsuitable for high value applications, in particular in the cosmetic industry, even though the existence of very fine grains urges people skilled in the art to seek an application that would make use of the advantages inherent in having silica powder supplied directly in the form of very fine grains.
There is therefore a need to whiten silica fumes so as to be able to make them more valuable.
However, the person skilled in the art encounters great difficulties with traditional incineration methods that are available, such as methods that make use of a rotating furnace or of a high-speed calcinator (also known as a "flash calcinator").
If a conventionally designed rotating furnace is used in the form of a rotating cylinder whose axis is at a small angle relative to the horizontal, with a flame being disposed at the inlet to or the outlet from the cylinder, it is observed that the substance injected into the cylinder to be turned over therein sticks together and reagglomerates into clusters of larger size, thus loosing its fineness.
This is explained by the fact that the two parameters of temperature and of contact time do not make it possible simultaneously to whiten the powder and to maintain the very fine grain size thereof. Since the contact time is de facto very short, either the temperature is selected to be quite low so as to avoid the particles sticking together, in which case the resulting powder is fairly gray in color, or else the temperature is raised in order to achieve satisfactory whitening, in which case the sintering temperature of the powder is exceeded and the resulting product is in the form of agglomerated lumps.
It would certainly be possible to provide an additional treatment system for reducing such agglomerated lumps to particles of a more acceptable grain size, but under such circumstances the method would be more expensive and in any event the geometry of the particles would be spoiled (crushed particles would be very rough and such lack of roundness would make them unsuitable for applications in which a very "smooth" powder is required, e.g. in the cosmetics industry as an ingredient in foundation make-up and face powder).
In addition, if a conventionally-designed flash calcinator is used which is in the form of a very tall narrow vertical cylinder with hot air being injected into the bottom thereof at high speed, although each grain of powder is individually supported to form an "entrained bed" and is subjected to a temperature that can be held below the sintering temperature (to avoid the particles sticking together), the powder remains in contact with high temperature air for too short a length of time (less than one second), such that the powder collected downstream from the calcinator is gray in this case also.
It would certainly be possible to increase the contact time by using a calcinator that is taller than the conventional height of existing calcinators (which is generally about 12 to 15 meters (m)), but that would lead to an enormous installation since it would have to be at least fifty meters tall.
Conventional methods thus do not make it possible to whiten silica fumes satisfactorily while retaining their very small grain size.
It should also be observed that the person skilled in the art would, a priori, avoid using any fluidized bed technique using hot gas with the powder to be treated since the grain size of the power is very much smaller than the limit of so-called "fluidizable" powders (generally 30 .mu.m to 50 .mu.m). Under such circumstances the grains subjected to inter-particle forces and to the force of gravity can no longer be supported by a fluidization flow and it is not possible to prevent the powder blowing away.
An object of the invention is to solve this problem by providing a method of treatment and apparatus for implementing the method that enable silica fumes to be whitened while preserving the fine grain size, which has not been possible with conventional methods, as explained above.
Another object of the invention is to provide a method and apparatus for implementing the method suitable for use under financial conditions that are reasonable, without requiring a treatment installation of exorbitant size, and with the treatment process being suitable for automatic control.
Another object of the invention is to provide a method and apparatus for implementing the method making use of means having a structure that remains simple and reliable, thereby making it easy to connect the treatment installation to the rest of the factory, with the feed of silica fumes and the collection of the finished product being automated.